I just happened to have the registration card for my '71 Starcraft at hand. It weighed just under 4500 lbs. when built OEM. It now weighs more than that. The tax base on the card says "4500". The registration tax is $8.00.
I asked similar questions when I just registered the old 2003 Impala my Son drives. It is value based and the value is listed at $21,000. I asked the counter person if MN would like to buy the vehicle at a reduced price. :B
Plate registration, titling, and annual renewals are antiquated but the State likes this system. It brings in a lot of money. Oh, to have Jesse Ventura back in office when registration was half of what it is now and we all got rebate checks from taxation.
The State now has a surplus in the coffers and legislators are tripping over each other, finding ways to spend it. They should all have to camp out one night under some failing highway bridge, IMO.